WebGL: Validate the read and write buffers for BlitFramebuffer are unique. TEST=conformance2/rendering/blitframebuffer-test BUG=angleproject:1990 Change-Id: I0caeaac824f1689867134f34f74e5ef2c2f1b016 Reviewed-on: https://chromium-review.googlesource.com/475990 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>